1. Field of the Invention
The invention relates generally to data communication networks, and more particularly to T-type connectors for connecting a network trunk line to network node devices via drop lines.
2. Description of the Background Art
Two types of connectors known in the art are T-connectors and attenuating node taps. T-connectors have been used to connect network node devices to networks of the type described in U.S. Pat. No. 4,319,338; U.S. Pat. No. 4,667,227 and U.S. Pat. No. 4,747,100, and known in the trade as Data Highway.TM. Data Highway II.TM. and Data Highway Plus.TM., respectively. Attenuating node taps are known for connecting node devices to carrier band networks specified under IEEE Standard 802.4.
The T-tap allows low cost node transceivers and low cost taps. However, due to the loading of the drop cable and node transceiver, signal reflections are set up on the network bus, thus distorting and attenuating network communication waveforms. To minimize signal reflections, very short drop cables are required (&lt;1/10 carrier wavelength), and the number of nodes, the node spacing and the maximum network length is restricted.
The attenuating tap uses transformer and resistor elements to match the impedance of all three ports. The advantages of the attenuating tap are reduction in signal reflections and an ability to use longer drop cables. A disadvantage of the attenuating tap is large attenuation in the signals which are transmitted and received. Expensive, high power transmitters and sensitive receivers are required in the node electronics to overcome this signal attenuation. Sensitivity to noise induced on the network is increased. Another disadvantage with the attenuating tap is the "through loss" of the tap is relatively high, thereby restricting the number of nodes and maximum network length.
Another T-connector device is illustrated in Glaab, U.S. Pat. No. 4,902,989. In this design the inductance of the transformer compensates for the capacitance in the drop cable, but this is not the full compensation needed for a node tap circuit.
A general problem in use of the above described networks is leaving one or more taps in the network disconnected from their load transceivers. This typically changes the impedance of the tap which can cause signal reflections and the undesirable results discussed above.